Field of the Invention
The present invention concerns a method to optimize a pulse sequence for operating a magnetic resonance system. Moreover, the invention concerns a method to operate a magnetic resonance system using such an optimized pulse sequence, as well as a pulse sequence optimization device and a magnetic resonance system designed to implement this method.
Description of the Prior Art
In a magnetic resonance system—also called a magnetic resonance tomography system—the body to be examined is typically exposed to a relatively high basic magnetic field (for example of 1, 5, 3 or 7 Tesla) with the use of a basic field magnet system. A magnetic field gradient is additionally applied by a gradient system. Via suitable antenna devices, radio-frequency excitation signals (RF signals) are then emitted from a radio-frequency transmission system, such radio-frequency excitation signals causing nuclear spins of specific atoms that have been excited to resonance by the radio-frequency field to be flipped by a defined flip angle relative to the magnetic field lines of the basic magnetic field. Upon relaxation of the nuclear spins, radio-frequency signals (magnetic resonance signals) are emitted that are received by suitable reception antennas and then processed further. The desired image data can be reconstructed from the raw data acquired in such a manner.
For a specific measurement (data acquisition), a defined pulse sequence is emitted, which includes a series of radio-frequency pulses (in particular excitation pulses and refocusing pulses) and gradient pulses emitted in coordination with the RF pulses. Readout windows, which temporally match these other pulses, must be set so as to predetermine the time periods in which the induced magnetic resonance signals are detected. The timing within the sequence—i.e. at what time intervals which pulses follow one another—is of particular significance for the imaging. Multiple control parameters are normally defined in a combination known as a measurement protocol, which is created in advance and can be retrieved from a memory (for example) for a defined measurement, and may possibly be modified by the operator on site. The operator may provide additional control parameters (for example a defined slice interval of a stack of slices to be measured, a slice thickness etc.). A pulse sequence (which is also designated as a measurement sequence) is then calculated based on all of these control parameters.
The gradient pulses are defined by their gradient amplitude, gradient pulse duration and edge steepness, namely the first derivative of the pulse shape dG/dt of the gradient pulses (typically also designated as “slew rate”). An additional important gradient pulse variable is the gradient pulse moment (also shortened to “moment”), which is defined by the integral of the amplitude over time.
During a pulse sequence, the magnetic gradient coils with which the gradient pulses are emitted are switched frequently and rapidly. Since the time requirements within a pulse sequence are usually very strict, and additionally the total duration of a pulse sequence that determines the total duration of an MRT examination must be kept as short as possible, gradient strengths around 40 mT/m and slew rates of up to 20 mT/m/ms must be achieved at least for short durations. In particular, such a high edge steepness contributes to the known noise development during the switching of the gradients. Eddy currents with other components of the magnetic resonance scanner (data acquisition unit)—in particular the radio-frequency shield—are one reason for this noise development. In addition, steep edges of the gradients lead to a higher power consumption and additionally impose greater demands on the gradient coils and the additional hardware. The rapidly changing gradient fields lead to distortions and oscillations in the gradient coils and to the transfer of these energies to the housing. In MR systems wherein the basic magnetic field is produced by a magnet having superconducting coils that are kept in a superconducting state by liquid helium, a high helium boil-off can occur due to heating of the coils and the additional components.
Particularly in order to reduce the noise exposure, various solutions have been proposed for the design of the hardware, for example casting or vacuum sealing of the gradient coils.